Exam1_S11 - ME 315 Midterm Exam 1 6:30 - 7:30 PM Thursday,...

ME 315 Midterm Exam 1 6:30 - 7:30 PM Thursday, February 10, 2011 •This is a closed-book, closed-notes examination. There is a formula sheet provided at the end of this exam. •You must turn off all communications devices before starting this exam, and leave them off for the entire exam. •Please write legibly and show all work for receiving partial credit. Please show your final answers in the boxes provided. •State all assumptions. •Please arrange all your sheets in the correct order. Make sure they are all included. Name: ____________________________________ Last First CIRCLE YOUR DIVISION Div. 1 (9:30 am) Div. 2 (12:30 pm) Prof. Choi Mr. Bao Problem Score 1 (25 Points) 2 (40 Points) 3 (35 Points) Total (100 Points)

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21. (25 points) A square glass window of width W= 1m is composed of three layers, as shown in the figure. The interior layer and outer layer are glass with thickness tg= 5 mm, while the intermediate layer is an air gap with thickness ta= 5 mm. Heat transfer through air gap is only by conduction. Thermal conductivity of glass and air are kg= 1.4 W/m⋅K and ka= 0.024 W/m⋅K, respectively. A thin coating is deposited on the inner surface of the interior layer and has an absorptivity of α= 0.1. The window is subjected to solar radiation flux G= 1.4 kW/m2. The glass and air may be assumed to be perfectly transparent, so that solar energy is only absorbed by the coating. Any other radiation effect, such as the radiation from the room and emission of the coating, can be neglected. Both side of the glass is subjected to convective heat transfer. The inside air temperature is T∞,i= 25 ºC, and the outside (ambient) has a temperature of T∞,o= 10 ºC and convection coefficient ho= 20 W/m2⋅K. At steady state, it is found that the coating has a temperature Tc= 40 ºC.

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